This application is based on and claims priority to Japanese Patent Application No. 11-366325 filed Dec. 24, 1999, the entire contents of which is hereby expressly incorporated by reference.
1. Field of the Invention
The present invention relates to a diagnostic system. More specifically, the present invention relates to a diagnostic system for a combustion condition sensor used with an engine.
2. Description of Related Art
In all fields of engine design, there is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued high or higher power output. In pursuit of better fuel economy and emission control, various types of control systems have been developed in conjunction with internal combustion engines. One of the more effective types of controls is so-called xe2x80x9cfeedbackxe2x80x9d control. With this type of control, a baseline or xe2x80x9creferencexe2x80x9d air fuel ratio is set for the engine. Adjustments are then made from the baseline setting based upon output from a combustion condition sensor that senses the air fuel ratio within at least one combustion chamber of an engine in order to bring the air fuel ratio into a desire range.
Normally, the type of combustion condition sensor employed for such feedback control is an oxygen (O2) sensor which outputs an electrical signal. When the output signal voltage is high, little oxygen is present in the exhaust, indicating that an air fuel charge combusted in the combustion chamber was xe2x80x9crichxe2x80x9d, i.e., more than the stoichiometric amount of fuel was present in the air/fuel charge. On the other hand, when the output signal voltage is low, substantial amounts of oxygen are present in the exhaust, thus indicating that a combusted charge was xe2x80x9cleanxe2x80x9d, i.e., less than the stoichiometric amount of fuel was present in the air/fuel charge.
A conventional oxygen sensor is normally connect to a wave forming circuit which manipulates the output of the sensor to indicate an xe2x80x9conxe2x80x9d signal when the voltage of the output signal exceeds a reference voltage (i.e., a signal which results when the supplied air/fuel charge is rich). On the other hand, the circuit manipulates the signal to indicate that the sensor is xe2x80x9coffxe2x80x9d when the voltage of the output signal does not exceed the reference voltage (i.e., a signal which results from a supplied air/fuel charge that is lean).
A control system incorporating such a sensor typically operates on a feedback control principal, continuously making corrections to accommodate deviations from the desired air/fuel ratio. Adjustments are made in stepped intervals until the sensor output goes to the opposite sense from its previous signal. For example, if the mixture is too rich in fuel (i.e., the sensor is xe2x80x9conxe2x80x9d) then the amount of fuel supplied to each fuel charge is reduced until the air/fuel ratio sensed is lean (i.e., the sensor signal turns xe2x80x9coffxe2x80x9d). Adjustments are then made back into the rich direction or back, thus approximately maintaining the desired ratio.
Most commonly, the oxygen sensor is the type which utilizes inner and outer platinum or platinum coated electrodes. However, due at least in part to the high operating temperature of such a sensor, the platinum acts as a catalyst, which catalyses the exhaust. For example, oxygen remaining in the exhaust may be catalyzed with carbon monoxide at the platinum electrode interface, creating carbon dioxide. Although the effects of the platinum in improving exhaust gas emissions may be advantageous, the oxygen content of the gas being sensed can be affected to a degree which causes the sensor to provide inaccurate data, causing the associated control system to adjust the air fuel ratio erroneously.
One aspect of the present invention includes the realization that diagnosis of a combustion condition sensor can be performed by monitoring certain operational parameters of the sensor. For example, the maximum and minimum voltage outputs of the sensor as well as the time elapsed during a transition between a maximum and minimum voltage output of the sensor can be used as an indication of the operational status of the sensor. Thus, by tracking such operational parameters of a combustion condition sensor, failure of the sensor can be diagnosed.
Accordingly, another aspect of the present invention includes a diagnostic system for an engine which includes a combustion condition sensor. The system comprises a controller which samples output from the combustion condition sensor and stores the output as a first output value. Subsequently, output from the combustion condition sensor is sampled to determine a second output value. The second output value is compared to the first output value in order to diagnose the combustion condition sensor.
Preferably, the combustion condition sensor is in the form of an oxygen sensor. In one mode, the first output value is a maximum voltage output from the oxygen sensor, which corresponds to the output of the oxygen sensor when the combustion of a xe2x80x9crichxe2x80x9d air/fuel charge is sensed. The second output value also corresponds to the detection of rich air/fuel combustion. The second value is compared with the first value to determine if the second value is higher than the first value. If the second value is higher than the first value, the second value is stored as the maximum output value of the oxygen sensor. Thus, the stored maximum output voltage of the oxygen sensor can then be used for diagnostic purposes of the oxygen sensor.
For example, a technician can read the stored maximum output value of the oxygen sensor and determine if the maximum output voltage is within a range corresponding to the proper operation of the oxygen sensor. If the maximum output voltage is outside the range corresponding to proper operation, the technician can then conclude that the oxygen sensor is malfunctioning and thus must be repaired or replaced.
In another mode, the first output value is a minimum output value of the oxygen sensor and the second output value is a subsequent minimum output value of the oxygen sensor. In a similar fashion, the controller stores the minimum output value of the oxygen sensor to determine if the oxygen sensor is operating properly.
In yet another mode, the controller can be configured to detect the transition of the sensor from a maximum voltage output to a minimum voltage output. This condition is caused, for example, when the air fuel charges combusted in the combustion chamber change from rich to lean mixtures, or from lean to rich mixtures. In this mode, the controller is configured to determine the time interval over which the oxygen sensor switches from the maximum output voltage to the minimum output voltage. The controller then stores this value as a transition time period. A technician can then compare the stored transition time period with a range of values that corresponds to the proper operation of the oxygen sensor. If the transition time period falls outside of the range, the technician can use this information to conclude whether the oxygen sensor might need repair or replacement.
The above-mentioned modifications are intended to be within the scope of the invention herein disclosed. These and other modifications of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment having referenced to the attached figures, the invention not being limited to any particular preferred embodiment enclosed.